1. Field of the Invention
This invention relates to an apparatus for monitoring the acceleration energy of an electron accelerator. In the accelerator a target is to be exposed to a beam of accelerated electrons, and an electron absorber is positioned behind the target. In particular, this invention relates to a monitoring apparatus for a linear accelerator applicable to the irradiation of patients.
2. Description of the Prior Art
With electron accelerators having a target exposed to the electron beam for the purpose of production of X-ray radiation, it is customary to measure the intensity of the emitted X-ray radiation with an ionization chamber positioned in the radiation path. The measuring signal of the ionization chamber is primarily dependent on the number of X-ray quanta which impinge into the measuring volume per time interval, and in much smaller degree, the measuring signal is also dependent on the energy of the X-ray quanta. The interaction of the X-rays with the gas of the ionization chamber increases as the quantum energy decreases. Given an unchanged number of X-ray quanta per time unit, dependency results in a small decrease of the measuring signal when the energy increases. Knowing the energy of the X-ray quanta or the energy of the electrons impinging on the target, it would be possible to assign the output signal of the ionization chamber to a definite number of X-ray quanta per time unit and volume unit. In medical applications, it is especially significant that as the quanta energy decreases, the interaction of the X-ray radiation with the body of the patient increases. Therefore, from the therapeutical aspect, the determination of the energy of the X-ray quanta, that is, of the hardness of penetrability of the X-ray radiation, is of special interest.
In electron accelerators having a target exposed to the electron beam for producing X-ray radiation, an electron absorber is conventionally located in the radiation path behind the target. The electron absorber consists of a material having an atomic number distinctly lower than that of the target, as for example, copper, aluminum, graphite. The electrons penetrating the target are absorbed in the electron absorber, without being able to produce afocal X-ray radiation. Behind the electron absorber, the X-ray radiation cone is free of electron radiation.
In electron accelerators it is also well known to build in the target in an electrically insulated way and to connect it to ground through a resistor. This has the advantage that in an inspection check of the accelerator, the charging curve of a target during a beam pulse can be recorded on a cathode ray tube. From the shape of the charging curve one can determine certain adjustment and tuning errors of the electron accelerator.